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<br />000278____ <br /> <br />location where it passes under the Main Outlet Drain Extension (MODE). Flows in the conduit <br />will consist of drainage water pumped from the Yuma Mesa Well field and from drainage wells <br />located in the Yuma Valley. <br /> <br />The trifurcation structure will be designed to allow drainage water from the Yuma Mesa conduit <br />to be discharged to three different locations. The drainage water could continue to flow by <br />gravity to the Colorado, or by use of gates and or pumps the drainage water could be discharged <br />directly into the MODE, or discharged to a pipeline which would allow the drainage water to <br />flow to the Yuma Desalting Plant. <br /> <br />Question 7: If the YDP is in operation, why is the trifurcation structure needed? <br /> <br />Answer: Use of the YDP for salinity control is expensive. Thus, the trifurcation structure is <br />being constructed to allow us some flexibility in controlling the salinity of flows at the NIB. <br />Should salinity levels at NIB become too high, making it impossible for us to meet the salinity <br />requirements spelled out in,minute 242, we would have the option of discharging a portion of the <br />drainage returns into the MODE, thus keep a portion ofthe drainage water out of the river above <br />the NIB and improving the salinity of flows delivered to Mexico when necessary. However, <br />should the return flow credits for this drainage water become important to Arizona or the United <br />States, the desalting plant could process a portion of the water carried by the Yuma Mesa <br />Conduit for delivery to Mexico above NIB, thus recovering the water for return flow credits and <br />at the same time reducing salinity levels in flows arriving at the NIB. <br /> <br />Question 8: Why would a portion of the water be diverted to the MODE rather than having all <br />of it go through the YDP or into the river for delivery to Mexico at the NII3? <br /> <br />Answer: Please refer to the answer for question number 7 above. <br /> <br />Question 9: What would be the maximum and minimum to be diverted to the MODE? <br /> <br />Answer: There is no set maximum or minimum flow that would be diverted to the MODE. <br />Such diversions would be dependent on the conditions that occur at the time. The principal <br />factors that would be used to determine how much water would be diverted to the MODE are the <br />salinity levels of the drainage returns in the Yuma Mesa Conduit, the amount of drainage flows <br />in the Yuma Mesa Conduit, salinity levels in the river at NIB, and salinity levels in the river at <br />Imperial Dam, and the need for return flow credits for Arizona and or the United States. One <br />would have to look at all of these parameters and determine on a case by case basis how much if <br />any water should be discharged into the MODE for salinity control at NIB. <br /> <br />Question 10: Question: Why is the historic maximum the appropriate level for the current <br />situation? <br />Answer: Figure 3 shows the volume of drainage pumping and the acres with shallow <br />groundwater in Yuma Valley from 1955 through 2000. From the graph, the decade of the 1970's <br />was the period-in which shallow groundwater was least. It is desired to restore groundwater <br />levels to approximately the levels of the 1970's. From the figure, the maximum pumping rate <br />